M2-PR1: LogicalPlanner skeleton + op_id/IdGen contract

[lmeyerov]

M2-PR1: LogicalPlanner skeleton + op_id/IdGen contract

Parent: #1139

Objective

Introduce the initial M2 LogicalPlanner spine with stable operator IDs and a pure planning contract from BoundIR to LogicalPlan.

Deliverables

• Add graphistry/compute/gfql/logical_planner.py with:
  • class LogicalPlanner
  • plan(bound_ir, ctx) -> LogicalPlan (pure function)
• Wire monotonic IdGen so every emitted LogicalPlan node receives a unique op_id: int.
• Add minimal covered planning paths sufficient to validate skeleton behavior.
• Preserve backend-neutral logical contracts (no executor selection in planner).

Acceptance Criteria

• LogicalPlanner.plan() is importable and returns a LogicalPlan root for covered shapes.
• Every emitted node has op_id, with uniqueness within plan.
• Determinism/purity tests prove BoundIR is not mutated and repeated planning is stable.
• Real binder->planner contract tests (not synthetic-only), including UNWIND-carrying fixtures.
• CI green.

Non-goals

• No full QueryGraph extraction (M2-PR2).
• No lowering cutover (M2-PR4).
• No optimizer passes.

References

• plans/compiler-refactor/milestone-M2.md
• plans/compiler-refactor/new-architecture.md

[lmeyerov]

Parent tracker: #1139.\n\nLane-specific testing expectations for PR1:\n- Planner determinism/purity tests (no BoundIR mutation).\n- op_id uniqueness + monotonic assignment tests across non-trivial plans.\n- Binder->planner contract test using real bound fixtures (not synthetic-only) to prevent metadata drift.